The present invention relates to display technologies.
Improvements in portable electronics has led to new designs that support low power consumption and miniature device sizes. In conjunction with the advent of improved portable electronics, light weight and low power displays are also being developed. Accordingly, the combination of smaller portable electronics and new displays has resulted in a variety of miniature display devices with a multitude of applications.
Prior art miniature display devices use miniaturized cathode ray tubes to display images. Cathode ray tubes, however, create numerous disadvantages. One disadvantage results from the power requirements of the cathode ray tube. In particular, a cathode ray tube typically includes an electrode beam that requires a large amount of power. To facilitate the large power requirements, extraneous power supplies are typically attached to the exterior of the miniature display device. The extraneous power supply, however, reduces the portability of the miniature display device.
Another disadvantage of using cathode ray tubes in miniature display devices results from the size requirements of the cathode ray tube. The design characteristics of cathode ray tubes requires minimum sizing of components. Specifically, the electrode beam used to generate images in the cathode ray tube must have a minimum displacement from the screen area of the cathode ray tube. Accordingly, the size requirements of the cathode ray tube make the cathode ray tube (CRT) impractical for use in most miniature display devices. For example, in miniature displays (such as head-mounted displays) where a wide range of peripheral vision and mobility is required the power and size requirements of cathode ray tubes make the use of cathode ray tubes impractical.
Spatial light modulators, such as liquid crystal displays, do not have many of the disadvantages of CRTs. Liquid crystal displays create an image by using an electric field to control the transmission of light through a liquid crystal.
FIG. 1 illustrates a prior art liquid crystal display. In particular, cell 100 shows a reflective liquid crystal display (xe2x80x9cLCDxe2x80x9d). Cell 100 includes a liquid crystal layer, 130, coupled between layer 105, layer 150, and four mechanical borders. For illustrative purposes only two mechanical borders, spacer 110 and spacer 140, are shown. The mechanical borders are used to maintain a predetermined space between layer 105 and layer 150. Accordingly, layers 105 and 150 in conjunction with the four mechanical borders are used to contain liquid crystal layer 130 and maintain the structural integrity of cell 100.
Cell 100 uses incident light, light source 105, to generate an image on layer 155. In one particular example of cell 100, light source 105 is polarized before it passes through layer 155 and enters liquid crystal 130 with a polarization plane at 45xc2x0 to liquid crystal 130""s molecular orientation. Subsequently, the polarized light is reflected on mirror 120 and returns through layer 155. In this example, the reflected light is transmitted through an analyzing polarizer as long as the polarization of the light is changed by the liquid crystal layer 130.
To generate an image, cell 100 changes the polarization of light passing through an area within liquid crystal 130. In particular, cell 100 includes a single transparent control electrode, layer 150 made from indium tin oxide. Cell 100 also includes an array of pixel electrodes (which may themselves form the mirror 120). Each electrode of the array of pixel electrodes, electrode 115, corresponds to a pixel of a generated image.
For example, to generate a single pixel image, cell 100 applies a voltage between a particular pixel electrode 115 and layer 150 using pixel electrode logic (not shown) and driver circuitry (not shown). The voltage difference between electrode 115 and layer 150 creates an electric field across liquid crystal layer 130. The field, in turn, changes the orientation of the molecules located in the liquid crystal 130, thus changing the polarization of light passing through the liquid crystal subjugated to the field. Accordingly, the changed polarized region of the liquid display layer 130 allows light from light source 105 to traverse from the mirror 120 across cell 100 and through the output analyzing polarizer (not shown). Thus, the contrast between the darkened and light regions of liquid crystal layer 130 creates the single pixel image on layer 155.
To generate a color image, cell 100 may use a time sequential color display system. In a time sequential color display system three light sources (e.g., a red light, a green light, and a blue lightxe2x80x94i.e. RGB light source) are sequentially illuminated upon liquid crystal layer 130. Cell 100 also includes synchronizing signals and logic (not show) that coordinate the transition between the different light sources and modulate the voltages applied to the array of transparent pixel electrodes. Using the three light sources, the synchronizing logic, and the synchronizing signals, cell 100 displays color images on layer 155.
The light weight and low power design characteristics of a liquid crystal display (xe2x80x9cLCDxe2x80x9d) makes the LCD ideal for use in head-mounted displays. For example, Provisional U.S. Patent Application No. 60/070,216, filed on Dec. 31, 1997, entitled xe2x80x9cAN IMAGE GENERATOR HAVING A MINIATURE DISPLAY DEVICExe2x80x9d describes a head-mounted display uses in conjunction with a LCD. Examples of specific LCDs that may be used in miniature display devices may also be found in U.S. patent application Ser. No. 08/801,994, filed on Feb. 18, 1997, entitled xe2x80x9cDISPLAY SYSTEM HAVING ELECTRODE MODULATION TO ALTER A STATE OF AN ELECTRO-OPTIC LAYER.xe2x80x9d The patent application entitled xe2x80x9cDISPLAY SYSTEM HAVING ELECTRODE MODULATION TO ALTER A STATE OF AN ELECTRO-OPTIC LAYERxe2x80x9d (Ser. No. 08/801,994, filed on Feb. 18, 1997) is hereby incorporated by reference.
Using a LCD in a head-mounted display provides advantages in terms of weight and power consumption, however, the use of LCDs in head-mounted displays also results in numerous disadvantages. One disadvantage results from the motion of the user wearing the head-mounted display. In particular, when the user""s head moves relative to the head-mounted display""s magnifying mirrors or lenses the user loses his/her field of vision. Accordingly, the user is unable to determine the circumference or perimeter of the liquid crystal""s active display area. The active display area defines the region of the liquid crystal display that is displaying a generated image. The active display area also defines the liquid crystal region and accompanying electrodes used to generate an image. Even when the user""s head does not move, the user may not realize that he/she is not seeing the entire active display area.
Another disadvantage results from using software to define an active display area in head-mounted displays. In particular, using software to define an active display area for a head-mounted display requires a specialized operating system. For example, software may place items within windows, and the software may create a border at the edge of the active display area. However, using software to define an active display area requires allocating pixel electrodes to define an active display area. The allocation of electrodes results in a loss of displayable image area.
A display device operable to generate images and a border image is disclosed. The display device in one example of the invention comprises an electrode structure coupled to a spatial light modulator display layer. The electrode structure is configured to define an active display area. The display of this example also comprises a first electrode surrounding the electrode structure and a second electrode coupled to the spatial modulator display layer. The second electrode is located above the first electrode, and a voltage difference between the second electrode and the first electrode is used to generate the border image.
In one particular example of the invention, the electrode structure is a rectangular array of pixel electrodes disposed on a single crystal silicon substrate. The second electrode is a transparent electrode, such as an Indium Tin Oxide (ITO) electrode on a cover glass which is above a nematic liquid crystal layer and the silicon substrate. The cover glass and the silicon substrate create a sandwich with the liquid crystal layer between the cover glass and the silicon substrate. The pixel electrodes form a mirror which reflects incident light back through the liquid crystal layer. A rectangular border electrode is also disposed on the silicon substrate and surrounds the array of pixel electrodes. The border electrode is also reflective and is separately controlled electrically so that its voltage (relative to the ITO layer) is set separately relative to the pixel electrodes. In this manner, an independently colored border may be set around the array of pixel electrodes by setting a voltage difference between the border electrode and the ITO layer independently of any of the voltages of the pixel electrodes.
In another example of the invention, a display device includes a border electrode structure which surrounds an active display area. The border electrode structure is operatively coupled to a light modulator which is capable of modulating light in response to an electromagnetic signal created by the border electrode structure.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.